1. Field of the Invention
This invention concerns chromium plated cutter elements for saws and other cutting instruments, including chainsaws. More specifically, it concerns a method of electroplating chromium metal on cutter element substrates.
2. General Background of the Invention
Many types of electrolytic plating solutions have been developed to deposit chromium electrochemically on a metal substrate. One of the most widely used solutions contains predominantly hexavalent chromium ions (CrVI), in the form of dissolved chromium trioxide (CrO.sub.3), which is mixed with water and a sulfate catalyst to produce a lustrous protective or decorative chromium plate. It has long been known that a predominantly hexavalent chromium ion solution produces a brighter, more lustrous thick plated product than a trivalent solution. Moreover, trivalent chromium ions have been considered undesirable in such solutions because they are thought to produce an ionic shield around the cathode in an electrolytic bath that inhibits electrodeposition of chromium. For these reasons, significant amounts of trivalent chromium have been considered an undesirable contaminant in chromium electroplating solutions.
More recently, U.S. Pat. Nos. 4,447,229 and 4,615,773 disclosed electrolytic plating bath solutions that contained both trivalent and hexavalent chromium. The current efficiency of these electroplating processes was improved by adding small amounts of methanol to a bath containing dissolved CrO.sub.3 electrolyte. This bath promoted rapid electrodeposition of a chromium plate, even in the absence of a catalyst, with greater uniformity of the plated product. Particularly good current efficiency was observed when the bath contained dissolved metallic ions, such as iron, Current efficiency was also enhanced by maintaining the pH at the cathode at about 2.0 with a metal ion buffer.
Although chromium plating processes have long been known, the versatility of industrial processes using such plating has been limited by the observation that chromium softens when heated. Such heat softening is a particular problem in production processes that plate chromium on a heat-hardenable substrate such as an alloy steel. In the production of cutter elements, for example, it is necessary to heat-harden an alloy steel substrate before electrochemically plating the substrate with chromium to avoid softening the chromium during a heat treatment step. The necessity of heating the substrate prior to plating introduces an additional costly step into the manufacturing process. The surface of the steel substrate oxidizes when heated and must be thoroughly cleaned with a caustic material or other cleaning agents prior to plating. If such a cleaning step is not performed prior to plating, the chromium metal does not adhere well to the underlying steel substrate.
Another drawback to conventional electrodeposited chromium plate is that hydrogen is evolved at the cathode and incorporated into the chromium metal. Hydrogen can then diffuse from the plated metal into an alloy steel substrate and may embrittle the metal alloy. The plated chromium can be heated to 500.degree.-650.degree. C. to evolve hydrogen avoid such embrittlement, but such heating unacceptably softens the chromium plate. Lower heat treatment temperatures can avoid chromium softening, but require prolonged periods of heating. Hence, prevention of hydrogen embrittlement of the substrate cannot be avoided by heat treatment without concomitantly sacrificing hardness of the chromium plate or prolonging the manufacturing process.
Yet another problem encountered in chromium electroplating is that conventional electrolytic baths contain high concentrations of hexavalent chromium ions, which are extremely toxic. The disposal of hexavalent chromium is subject to strict and costly environmental regulations that greatly increase the expense of electroplating processes. Although it would be desirable to reduce the amount of hexavalent chromium ions in the bath, such reduction has been considered unadvisable because it produces a dull product which is not suitable for decorative or engineering purposes. The trivalent species has also been considered a contaminant in a predominantly hexavalent bath. U.S. Pat. No. 4,615,773, for example, required neutralization of trivalent chromium in a hexavalent solution to allow electroplating to occur. In view of the belief that trivalent chromium is an unwanted contaminant, the amount of hexavalent ion used in electrolytic solutions has not been decreased. Moreover, large amounts of methanol have not been added to chromium electrolyte baths because the methanol was known to produce trivalent chromium ions, as in U.S. Pat. No. 4,447,299.
It is an object of this invention to provide a process for electrolytic deposition of chromium that is environmentally safer than previous processes.
Another object of the invention is to provide such a process that can eliminate the necessity for cleaning oxidation products produced by heating a substrate before electroplating.
Yet another object is to provide such a process that produces chromium plated cutters which harden or maintain their hardness when heated, and display excellent wear characteristics.
Finally, it is an object of the invention to provide a product having superior adhesion between the chromium plate and underlying substrate.
These and other objects of the invention will be understood more clearly by reference to the following detailed description and drawings.